Paper web coating is today implemented using a variety of different methods. Short dwell time coaters are employed particularly as on-machine coaters due to their good running properties. Good running characteristics are based on a low application pressure and a short distance between application and doctoring. In short dwell time application, the coating mix cannot penetrate deeply into base web, the required doctoring force remains low and, due to the short time interval between application and doctoring, wetting of the base web and resulting loss of base web strength is less than in other coaters.
In the art, the "short dwell time coater" is generally denoted by the abbreviation SDTA (Short Dwell Time Applicator). In this type of coater, the coating mix is passed into an application chamber located immediately to the rear of the doctor blade, whereby one wall of the chamber is formed by the moving web supported by a backing roll, one wall by the doctor blade and one wall on the incoming side of the web by the front wall of the chamber. The coating mix is fed into the application chamber via a feed slit disposed at the bottom of the chamber, and a major portion of the infed mix is routed past the front wall edge back for coating mix recirculation. The purpose of the outflow of the coating mix past the front edge, counter to the web travel direction, is to reduce the access of the air layer travelling along with the running web into the application zone, to assure even distribution and replenishment of the coating mix flow and to keep the mix outflow slit open. Such a coating apparatus is described, for example, in U.S. Pat. No. 4,350,211.
To achieve an even surface profile of the web, some kind of coat profile control may be required. Such need for control is typically dictated by the unevenness of the base paper web profile or various factors related to equipment employed and by prevailing process conditions. Conventionally, the profile control is implemented by altering the doctor blade settings and a number of different control methods and equipment are known in the art.
U.S. Pat. No. 4,405,661 describes a coater of the above-mentioned type having the distribution of the coating mix in the application chamber as well as the evenness of application improved by a specific arrangement of the mix feed slit. In this apparatus the doctor blade is loaded by a pressurized loading hose, and the linear loading of the blade is controlled by adjusting the hose pressure and adjustment screws attached to the hose. The cross-machine profile of the doctor blade can be adjusted within certain limits by altering the control screw settings. The distance of the front edge on the incoming side of the web and the web is adjusted by a servo-controlled screw. However, this apparatus offers rather limited control capabilities of the coat profile because the pressurized hose absorbs a portion of the profile adjustment exerted by means of the control screws. U.S. Pat. No. 5,109,792 discloses an apparatus in which an improvement in coat quality is attempted by way of tighter sealing of the application chamber. This goal is achieved by means of an elastic seal element attached to the front edge of the chamber. The purpose of the seal element is to prevent the backflow of the coating mix and thus increase the internal pressure of the application chamber. This apparatus offers reduced backflow combined with lower risk of damage to the backing roll in the case of a web break. The doctor blade loading and profile control are implemented in the same way as in U.S. Pat. No. 4,405,661.
Finish Patent No. 91,025 describes a method for measuring the coat profile and adjusting the doctor blade profile in a coater. According to this method, the cross-machine profile of the amount of the coating mix applied on to the web in the coating station is measured continuously, and, on the basis of the measurement result, the blade profile is adjusted by means of elements controlling either the blade directly or via the blade support beam. This method is not limited to any particular doctor blade construction, but rather, is suited for controlling a variety of equipment having a doctor blade profile adjustment facility.
Coat profile control based on doctor blade profile control and doctor blade loading adjustment has several shortcomings. For instance, as the doctor blade loading changes with the adjustment of the blade profile, uneven doctor blade wear results. Such uneven wear causes a need for more frequent blade replacement particularly when blade profile control must be used correcting a coat profile error caused by a persistent defect of the base paper web. In such a case, the coater must be run with the same control settings for long period of time, whereby blade wear will eventually reflect in the controlled blade profile. A similar situation occurs if the profile control must be used to compensate for constructional errors of the applicator apparatus such as deflections of the blade support beam. Due to the resulting blade wear, the controlled profile will not remain stable over a longer period of time, but rather, relatively rapidly tends to assume the condition prevailing prior to the profile control operation thus necessitating frequent readjustment of the blade profile. Hence, methods based on blade profile control are not particularly desirable due to the short-term effect of the applied control measures.
Profile errors traceable to the base paper defects may also be corrected by calendering the web prior to coating. Naturally, while calendering improves paper quality, it also requires higher investments in equipment.